The external pixe milliprobe at davis: laser alignment, pixe calibration, and quality assurance

Abstract

An external beam PIXE system has been developed at the Davis cyclotron, which is being used extensively for the analysis of the inks and papers of historical documents. This system has several important features. First, because the system is used in valuable and fragile documents, we have kept the heat loss very small. The amount of beam on the sample is minimized by a close detector geometry and by on-demand beam pulsing. A computer driven beam stop allows beam on the sample only when the data is being acquired. Second, the 1 to 2 mm 2 beam spot can be accurately and quickly located at the desired position using a laser alignment system. With this system we can analyze single letters on one side of a page. Third, the beam on sample is internally monitored using the argon present in the 3 cm gap between the exit window and the sample. This is essential for thick targets which stop the beam. Finally, the elemental results are corrected for the decrease of beam energy at the target and for the X-ray attenuation between the target and the detector, and verified by thin standard foils. Similar corrections are made for the finite thickness of the samples. Multiple measurements on the inks of a single page show good consistency, whether done during the same cyclotron run or at another time. Our work indicates that external beam PIXE can be a very safe and accurate tool in historical research.